As is well-known, diesel engines, being compression-ignition engines, need to compress a fuel-air mixture so that it reaches a temperature adequate to cause its ignition. During this compression, if not done rapidly, various leakages become more significant, and heat of compression is lost to the piston, cylinder walls and head of the engine, if these parts are not already adequately heated. Such problems arise upon starting a cold diesel engine. To compensate for the heat of compression that is lost under these conditions, glow plugs are provided to supply additional heat to the combustion chamber, to facilitate successful ignition of the fuel when it is sprayed by an injector into the heated air.
As will be apparent, some type of controller is necessary to turn glow plugs on and off at appropriate times, and under appropriate conditions, to give an operator an indication of when the engine may be easily started, and for insuring that the glow plugs become warm enough to facilitate starting, and not excessively warm, leading to premature failure or shortened lifetime.
There are two types of glow plugs in general use, the linear resistance type, and the positive temperature coefficient (PTC) type. The linear resistance type is unable to play any part in controlling its own temperature, although previously the same controllers have been used for both linear resistance and PTC type glow plugs. In the case of PTC type glow plugs, their resistance increases with temperature, limiting the current flow therethrough, and limiting the resultant temperature. Such a prior art controller is disclosed in U.S. Pat. No. 4,177,785, issued to Sundeen on Dec. 11, 1979. This patent also discloses the preferred glow plug temperatures, and the general structure and operation of a diesel engine as related to starting such an engine. As set forth therein, glow plugs may either be operated at their rated voltage, or may be operated above their rated voltage by cyclically completing and interrupting the glow plug heater element energizing circuit with a bimetallic element carefully matched to the thermal characteristics of the glow plugs. This arrangement requires a thermally operated circuit breaker that must be carefully designed so that it does not operate before the bimetallic element which cyclically energizes the glow plugs, imposing a design limitation which is difficult to meet over a large range of ambient temperatures, so that at one ambient temperature the circuit breaker may fail to operate in an appropriate time to protect the system from damage, while at another ambient temperature it may operate prematurely, removing power from the glow plugs before an adequate temperature has been reached.
Also, previous such control devices were either constructed as two separate assemblies, one assembly containing the control circuitry and a separate second assembly including a power relay, or had both the control circuitry and power relay in a common package, which did not isolate the control circuitry from heat generated by the coil and contacts of the power relay, so that, for instance, heating due to contact deterioration in the power relay, or resistive heating of its coil, would be detected as the equivalent to an increase in glow plug temperature, leading to a glow plug temperature below that considered to be sufficient for reliable starting. If placed in a common package, the entire package was discarded upon failure of any part of the device.